The objective of this pilot research is to further refine and evaluate a newly devised intervention to alter young cancer patients' capacity for, and level of involvement in, physical and social activity, and to begin to examine the extent to which this intervention will favorably affect long-term adjustment to cancer and its treatment. We hypothesize that the physical/social activity program we are developing, offered as an intervention to adolescents who have recently completed therapy for cancer, will positively affect their physical and psychological adjustment to their disease. Adolescents who receive the intervention will be more likely to experience better physical and social functioning, good quality of life and enhanced post-traumatic growth, and less likely to experience post-traumatic stress, than those who do not. Those who are required by nature of their disease, its treatment, or its physical and psychological sequelae to substantially reduce activity in either or both domains, and who do not receive the experimental intervention will have a greater likelihood of experiencing more negative outcomes, with declines in physical capacity/involvement and social involvement, a decreased quality of life and a greater likelihood of post-traumatic stress, as opposed to posttraumatic growth or resilience. The study utilizes a randomized, between groups, repeated measures design, with an Intervention Condition (1C) and a Control (standard care/wait list) Condition (CC). An initial development phase involving six subjects will allow further refinement of the intervention and measures, prior to start of the intervention trial. For the intervention trial, 44 12-16 year old youths, at the end of their treatment for leukemia or brain tumors will be recruited. Using a randomized, between-groups, repeated measures design, 22 subjects will be randomly assigned to either the Intervention or the Control Condition. Subjects assigned to the 1C will receive a 12-week long aerobic exercise and physical activity program and will receive training in social skills, provided to groups of four-six. A multi-informant, multi-method assessment of both physical and social activity will be obtained at Baseline, at the end of the intervention, and at three3-month follow-up during both the development and intervention phases. Assessments will include self- and parent-proxy ratings of physical and social functioning, objective and self-report measures of activity level, cardiopulmonary exercise testing and hemoglobin/hematocrit levels. Self- and parent-proxy ratings of post-traumatic stress symptoms, posttraumatic growth and quality of life will also be obtained, as indicators of both positive and negative adjustment to their disease. Subjects in the 1C will be compared to an equal number of subjects enrolled in a standard care CC on identical outcome measures.